1. Field
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred on (part of) the substrate (e.g., a glass plate), e.g., via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
Instead of a circuit pattern, the patterning means can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or which covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through the projection beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
When using a print head comprising an array of individually controllable elements, the print head may not extend across an entire width of the substrate. Thus, several print heads are used or the substrate is shifted relative to the print head once exposure of a first area has been completed. Furthermore, the pitch between individually controllable elements used in the print head can be much larger than the required pitch between spots exposed on the substrate. It can be difficult to fit sufficient print heads into the limited space available, so generally a combination of these two methods is used and the substrate is shifted relative to the plurality of print heads. However, the alignment of print heads with respect to each other may not be completely accurate. Furthermore, the shift of the substrate may not be exactly correct.
Both of these lead to so-called stitching errors due to the misalignment of printed pixels in the exposed areas. Although steps can be taken to reduce stitching errors, these all occur in the same area where the exposures from a first exposure by a first print head overlap with any other exposures. The spatial frequency of these areas of stitching can be visible to the human eye, thus reducing the image quality.
Therefore, what is needed is a system and method of exposing the substrates using arrays of individually controllable elements in which the effect of stitching is minimized.